flake: Build variants without tests

Tests set the C++ version to 17, however only up to 11 is allowed in the
other files.

flake.nix

@@ -7,6 +7,10 @@
   outputs = { self, nixpkgs, flake-utils }: flake-utils.lib.eachDefaultSystem (system:
     let
       pkgs = import nixpkgs { inherit system; };
+      mkNocheck = drv: drv.overrideAttrs (o: {
+        doCheck = false;
+        nativeBuildInputs = pkgs.lib.filter (p: p != pkgs.catch2_3) o.nativeBuildInputs;
+      });
     in
     {
       packages = rec {
@@ -33,6 +37,9 @@
             doCheck = true;
           })
           { };
+
+        u01-nocheck = mkNocheck u01;
+        u02-nocheck = mkNocheck u02;
       };
 
       devShells.default = pkgs.mkShell {